The design of early integrated circuits focused on implementing an increasing number of small semiconductor devices on a semiconductor substrate to achieve substantial improvements in manufacturing efficiency and cost, product size, and performance. The continuing improvements in the design of integrated circuits over the past few decades has been so dramatic and so pervasive in numerous products that the effects can be measured in changes in industries.
The design and construction of integrated circuits has continued to evolve in a number of different areas. One area of innovation is a continuing reduction of feature sizes of semiconductor devices such as control and signal processing devices formed on a semiconductor substrate. Another area of innovation is the advent of construction techniques to incorporate higher voltage semiconductor devices (also referred to as “higher voltage devices”) having higher voltage handling capability such as switches of a power train of a power converter into the integrated circuits.
An objective of incorporating control and signal processing devices on a semiconductor substrate with the higher voltage devices often encounters conflicting design requirements. More specifically, lower voltages (e.g., 2.5 volts) are employed with the control and signal processing devices (hence, also referred to as “low voltage devices”) to prevent flashover between the fine line structures thereof. A potential difference of only a few volts separated by a fraction of a micrometer can produce electric fields of sufficient magnitude to induce locally destructive ionization in the control and signal processing devices.
When employing the higher voltage devices therewith, it is often necessary to sense and switch higher external circuit voltages (e.g., ten volts or higher) on the integrated circuit. To accommodate the higher voltage devices on a semiconductor substrate with the control and signal processing devices, a large number of processing steps are performed to produce the integrated circuit. Since the cost of an integrated circuit is roughly proportional to the number of processing steps to construct the same, there has been limited progress in the introduction of low cost integrated circuits that include both control and signal processing devices and higher voltage devices such as the switches of the power train of a power converter.
The aforementioned constraints have been exacerbated by the need to employ a substantial area of the semiconductor substrate to incorporate more efficient and even higher voltage devices into an integrated circuit. Inasmuch as the cost of a die that incorporates the integrated circuit is roughly proportional to the area thereof, the presence of the higher voltage devices conflicts with the reduction in area achieved by incorporating the fine line features in the control and signal processing devices.
With respect to the type of semiconductor devices readily available, complementary metal oxide semiconductor (“CMOS”) devices are commonly used in integrated circuits. The CMOS devices such P-type metal oxide semiconductor (“PMOS”) devices and N-type metal oxide semiconductor (“NMOS”) devices are used as logic devices, memory devices, or other devices such as the control and signal processing devices. In addition to the CMOS devices, laterally diffused metal oxide semiconductor (“LDMOS”) devices such as P-type laterally diffused metal oxide semiconductor (“P-LDMOS”) devices and N-type laterally diffused metal oxide semiconductor (“N-LDMOS”) devices are also commonly used in integrated circuits. LDMOS devices are generally used for the higher voltage devices in the integrated circuit. In the context of CMOS technology, the higher voltage devices generally relate to devices that operate at voltages above a standard operating voltage for the selected CMOS devices (e.g., the low voltage devices). For instance, CMOS devices employing fine line structures having 0.25 micrometer line widths operate at or below about 2.5 volts. Thus, higher voltage devices generally include any devices operating above approximately 2.5 volts.
Integrating the CMOS and LDMOS devices on a semiconductor substrate has been a continuing goal in the field of microelectronics and has been the subject of many references over the years. For instance, U.S. Pat. No. 6,541,819 entitled “Semiconductor Device Having Non-Power Enhanced and Power Enhanced Metal Oxide Semiconductor Devices and a Method of Manufacture Therefor,” to Lotfi, et al., issued Apr. 1, 2003, which is incorporated herein by reference, incorporates non-power enhanced metal oxide semiconductor devices (i.e., low voltage devices) with power enhanced metal oxide semiconductor devices (i.e., higher voltage devices) on a semiconductor substrate. While Lotfi, et al. provides a viable alternative to integrating low voltage devices and higher voltage devices on the semiconductor substrate, further improvements are preferable in view of the higher voltage handling capability associated with the use of higher voltage devices such as with the LDMOS devices in the power train of a power converter.
In the field of power microelectronics, the CMOS devices may be employed as the control and signal processing devices integral to the controller of a power converter. As an example, the control and signal processing devices are employed as low voltage switches and comparators that form portions of the controller of the power converter. The LDMOS devices, on the other hand, may be employed as the higher voltage devices integral to the power train of the power converter. The higher voltage devices perform the power switching functions to control the flow of power to, for instance, a microprocessor. The power switches include the main power switches, synchronous rectifiers, and other power switches germane to the power train of the power converter. The power switches can also be used for circuit protection functions such as a rapidly acting electronic version of an ordinary fuse or circuit breaker. Variations of power switches include metal oxide semiconductor field effect transistors (“MOSFETs”) that exhibit low level gate-to-source voltage limits (e.g. 2.5 volts) and otherwise are capable of handing the higher voltages germane to the power train of the power converter.
To achieve the overall reduction in size, the integrated circuits as described herein should include control and signal processing devices with fine line structures having sub micron line widths (e.g., 0.25 micrometers) on a semiconductor substrate that operate with lower voltages to prevent flashover within the integrated circuit. At the same time, the integrated circuit may incorporate higher voltage devices that can conduct amperes of current and withstand voltages of, for instance, ten volts. A benefit of incorporating the low voltage devices and the higher voltage devices on the semiconductor substrate is that it is possible to accommodate higher switching frequencies in the design of the power processing circuit due to a reduction of parasitic capacitances and inductances in the integrated circuit.
While a design and implementation of low voltage devices such as logic devices that form portions of a microprocessor have been readily incorporated into integrated circuits, the systems that power the logic devices have not to date been readily incorporated into integrated circuits. There has been pressure directed to the power electronics industry to make parallel improvements in the power conversion technology and, in particular, with the power converters that regulate the power to, for instance, the microprocessors that employ a high level of integrated circuit technology in the design thereof. Thus, an evolutionary direction in the power electronics industry is to reduce the size and cost of the power converters which correspondingly induces greater levels of silicon integration in a design of the integrated circuits embodying the same.
Although power converters have shown dramatic improvements in size, cost, and efficiency over the past few decades, the design of the power converters have not kept pace with the improvements in integrated circuit technology directed to the logic devices and the like, which follow Moore's Law demonstrating a doubling of performance every 18 months as viewed by certain metrics of digital performance. As representative examples of improvements in the smaller and more compact power converters, see U.S. Pat. No. 5,469,334, entitled “Plastic Quad-packaged Switched-mode Integrated Circuit with Integrated Transformer Windings and Mouldings for Transformer Core Pieces,” to Balakrishnan, issued on Nov. 21, 1995, and U.S. Pat. No. 5,285,369, entitled “Switched Mode Power Supply Integrated Circuit with Start-up Self-biasing,” to Balakrishnan, issued on Feb. 8, 1994, which are incorporated herein by reference. While Balakrishnan and other references have demonstrated noticeable improvements of incorporating power converters into an integrated circuit, an industry wide integration of higher voltage level devices (again, such as the switches of the power train) into the design of integrated circuits, especially in power converters, has not yet gained industry wide adoption.
Another issue in a design of the power converters is an increase of the switching frequency (e.g., five megahertz) of the power train thereof. The energy stored in reactive circuit elements (e.g., inductors and capacitors) associated with the power converter is inversely proportional to the switching frequency, and the size of the reactive circuit elements is also correspondingly inversely proportional to the switching frequency. A power converter is generally designed to handle the highest switching frequency without significantly compromising power conversion efficiency. Otherwise, the switching frequency could be simply increased with a consequent reduction in the size and cost of the power converter. Achieving a high switching frequency is dependent on reducing the parasitic circuit elements such as stray interconnection capacitance and inductance. As mentioned above, incorporating the low voltage devices and the higher voltage devices within an integrated circuit embodying the power converter can have a significant impact in reducing the interconnection paths and consequently the stray interconnection parasitic capacitance and inductance. Additionally, reducing the inherent parasitic losses in the switches of the power converter such as energy stored in a gate of a MOSFET can also have a significant impact on the switching frequency of the power converter.
Accordingly, what is needed in the art is an integrated circuit and method of forming the same that incorporates higher voltage devices and low voltage devices on a semiconductor substrate that overcomes the deficiencies in the prior art. Additionally, there is a need in the art for a higher voltage device (e.g., a transistor such as a LDMOS device) that can accommodate higher voltages and is capable of being integrated with low voltage devices on a semiconductor substrate in an integrated circuit that may form a power converter or portions thereof.